Solenoid fuel injectors are commonly used in internal combustion engines. As it is well known, in solenoid actuated fuel injectors a solenoid coil is associated with a pintle assembly that cooperates with an outlet orifice at the injector tip to open or close the latter. The injector is configured such that when the solenoid coil is energized, it generates a magnetic field that allows lifting the pintle off its sealing seat at the injector tip, and thus causes the flow of fuel through the outlet orifice. When the solenoid coil is de-energized, the pintle assembly returns onto its seat under the action of a return spring and pressure acting thereon.
Modern developments of solenoid fuel injectors have led to high switching speeds. But the downside thereof is high impact velocities of the needle assembly on the valve seat, which causes noise, wear and fatigue, as well as bouncing of the pintle assembly at closing.
Pintle bouncing is particularly critical as it causes multiple parasitic injections, which reduce injection precision and deteriorates emission and efficiency. This contrasts with current and future emission legislation limits together with the demand for low fuel consumption that hence implies a more effective combustion in modern automotive engines.
Besides, in some uses, for example for gaseous fuel injection, wear of the valve seat due to needle impact is a major concern. Indeed, the wear phenomenon is more critical due too poor lubrication capabilities of gaseous fuels.
Nevertheless, improvements in the switching behavior of solenoid injectors, with shorter opening and closing times, by means of electronic control strategies show significant potential. Therefore, mechanical and electronic solutions have been developed to reduce bouncing.
Bouncing can be reduced by introducing hydraulic flow resistance into the fuel support. This leads to a limitation of upper injection volume per time and affects the final application. A controlled anti-force from the braking current in the coil after lift off can compensate excessive spring force and is able to almost completely eliminate bouncing. However, the system is sensitive to parameter variation, which makes it difficult to apply in practice.
Studies have shown that the major parameter affecting open-loop control of the braking current is fuel pressure. In this connection, it has been suggested that needle velocity information could be used as a parameter to determine optimum braking current parameters (such as trigger timing, duration, amplitude). This being said, it is desirable to have reliable means for determining needle velocity without any dedicated sensor.